Complexes based on trans-ReO2+ exhibit long-lived emission from the 3E(g) ligand-field excited state. This emission is quenched by proton transfer from a variety of acids. Reported here are results of emission quenching of ReO2L4+ (L=py, 3-Cl-py, 4-OMe-py) by common nitrogen and oxygen acids. Marcus analysis of the free energy dependence of the quenching rate constants points to a low barrier to proton transfer (lambda approximately 4 kcal). The results of the analysis can be used to calculate a pK(a)* of 11.3 for the py complex, which is consistent with earlier results for metal hydride acids. For the py and 3-Cl-py complexes, the standard Marcus relation can be used; however, fitting of the data for the 4-OMe complex requires the introduction of an asymmetry parameter, epsilon, which is related to the degree of excited-state distortion in the chromophore. Comparison of the excited-state Re-O stretching frequency obtained from low-temperature absorption spectra with the corresponding ground-state frequency obtained from resonance Raman spectra shows that there is a larger excited-state distortion in the 4-OMe complex.